U.S. Pat. No. 4,901,558 discloses a method of leakage testing containers having a wall structure which is flexible at least at some areas. The containers are thereby placed into a vacuum chamber and latter is evacuated and the bulging of the flexible wall areas of the containers is evaluated as measured value or signal and depending from their behaviour the determination of the presence of a leakage or tight state is made.
The disclosure refers also to problems which occur when the containers tested in this way are filled by contents which clog prevailing leakage areas. Apart from the contents the structure of such containers, such as containers having cardboard walls which at the inside have a plastic coating can lead to the fact that leakage areas obtain a valve-like characteristic, i.e. when a pressure differential is applied across the wall directed from the inside towards the outside these leakage areas tend to act in a self-sealing manner. Inspite of such, however, also such leakages cannot be tolerated because they lead to a spoiling of the contents.
Specifically the technique disclosed in mentioned U.S. Pat. No. 4,901,558 is detrimental for such containers or containers filled with such contents, because the pressure differential during the testing acts from the inside towards the outside due to the fact that a vacuum is applied around the container.